1. Field of the Invention
The present invention concerns a process and device for scavenging of a two-stroke internal combustion engine, with a linearly reciprocating piston, of the type autosupercharged by afterfilling effect, in which the distribution of each cylinder is exclusively assured by the piston in combination with the inlet port-inlet duct set and with the exhaust port-exhaust duct set, the inlet port set being exposed by the piston for a longer time than the exhaust port set, and a two-stroke engine provided with such scavenging.
More specifically, the invention is essentially aimed at a process and device for arrangement of the outflow for the temporary back-scavenging of the gases burnt in the inlet ducts during the pre-exhaust phase preceding opening of the exhaust ports, then for their reintroduction followed by introduction of the scavenging air proper inside the cylinder to improve the output of such an engine. All the air reaching the cylinder, whether for scavenging or for afterfilling, consequently enters by the inlet duct. This type of engine autosuper-charged by afterfilling effect utilizes the direct action of the gases emerging from one cylinder on an air supply for its compression and introduction into one cylinder of an opportune instant in the cycle to supercharge it. The phases of cylinder-air supply-cylinder pressure transfer operate in an essentially closed environment in such a manner as to avoid any energy loss to the outside.
Afterfilling is thus understood to mean in the introduction of an additional air charge into the cylinder, after scavenging and closure of the exhaust apertures.
It will be generally appropriate below to describe scavenging air as being the scavenging agent intended to renew the burnt gases of the cylinder, whether this be pure air or any other combustion supporter-fuel mixture.
2. Description of the Prior Art
Engines of the aforesaid type are already known, notably such a two-stroke engine, constituted by at least one set of two cylinders, whose operating cycles are wedged in relation to each other at a crank-shaft angle of 180.degree., in which the energy of the pre-exhaust gases of one cylinder is utilized to accomplish afterfilling of the other cylinder, is described in FR-A-s 346 558 or its equivalent CH-A-593 420 of the present applicant, and represented in FIGS. 1-5 of this publication.
Another process described in French Patent Application No. 87 04757 of the present applicant also presents an engine of the aforesaid type accomplishing autosupercharging of each cylinder by itself by an afterfilling effect through the use of an air supply pressurized by the action of the energy of the pre-exhaust gases.
A preferred version of the device for embodiment of the process also forms the object of a PCT-form co-application filed by the inventor under No. PC-FR 88/00155 published under No. WO 88/08073.
To facilitate continuation of the description, it will be appropriate to understand the term "double-cylinder solution", the latter forming the object of FR-A-2 346 558 or CH-A-593 420, as meaning that it may also be applied to a multiple of a set of two cylinders, and the term "single-cylinder solution", the latter being intended to embody the process according to French Patent Application No. 87 04757, as meaning that it may be applied in a universal manner, whatever the number and arrangement of the cylinders.
Before any description of the constraints specific to this particular type of two-stroke engine, it is appropriate to recall the purpose of scavenging in the two-stroke engine in general; its purpose is to be able to expel from the cylinder the gases burnt in the preceding cycle:
1) with the smallest possible quantity of scavenging air;
2) as completely as possible;
3) with the smallest possible scavenging air loss;
4) with the smallest possible energy expenditure, applied for this effect;
5) within the allotted time imposed by the engine closure conditions;
6) with intense as possible cooling of the cylinder and piston head;
7) benefiting, if possible, from the natural descending and then ascending motion of the piston to maintain the aerodynamics of scavenging.
Scavenging of an engine of the specific type described above and forming the object of the invention while satisfying in the best way possible general constraints 1)-7) noted above should moreover simultaneously resolve a set of particular constraints specified below. These supplementary particular constraints are inherent in the principle of operation of both the double-cylinder solution and single-cylinder solution and are as follows:
8) to organize temporary back-scavenging of the gases burnt in the scavenging ducts, then their return in the cylinder direction in such a manner that they mix as little as possible with the scavenging air, which excludes the possibility of imposing on the scavenging ducts a shape using excessively small radii of curvature;
9) to accomplish a form of scavenging inside the cylinder with the aid of the exhaust gases emerging from the scavenging ducts and then with the aid of the scavenging air front proper consistent with criteria 1)-7) noted above in a manner equivalent to what is obtained in engines of the conventional type where the inlet ports are exposed by the piston for a longer time than the scavenging ports, i.e. through the creation of a scavenging air front as unified and as concentrated as possible in such a manner as to avoid the creation of islets of burnt gases inside the scavenging zone (difficulty of synchronizing the return of several scavenging air fronts emerging from different ducts with different individual length and/or volumes);
10) an as reduced as possible interaxis distance between the cylinders for the double-cylinder solution or for the single-cylinder solution applied to a multi-cylinder engine;
11) to provide the scavenging ducts, in which the phenomenon of pressure exchange is to progress in a closed environment for the cylinder-scavenging duct-supercharged volume system (associated cylinder wedged at 180.degree. for the double-cylinder solution and piston internal air supply for the single-cylinder solution), with an optimum volume to achieve the maximum afterfilling effect. The theoretical optimum volume (the pre-exhaust gases then occupying a volume equal to that of the scavenging ducts) in actual fact depends on the extent of the envisaged afterfilling effect, which itself depends on the thermo-dynamic state of the gases under expansion (factor highly dependent specifically on the air-fuel ratio and the rate of the residual gases with which combustion has taken place). The true optimum volume of the scavenging ducts should naturally be greater than the theoretical optimum volume to avoid any risk of afterfilling by the burnt gases but also as little as possible, lest there should be any impairment, on the one hand, of the autosupercharging potential (decrease in the pressure level attained in the noted ducts if the energy supplied by the pre-exhaust gases is distributed over an excessive volume) and, on the other, of the engine output (increased loss of expansion in the cylinder without any beneficial effect on the increase in afterfilling thus obtained);
12) to provide the noted scavenging ducts with a length necessary for the creation of the optimum volume as explained in 11) above, but as small as possible in such a manner as to maximize the engine conditions, at which an afterfilling effect would still be just attained (any increase in the conditions causes a quasio-proportional increase in the passage time, expressed in crankshaft degrees, of the pressure wave due to expansion of the pre-exhaust gases).
The difficulty of simultaneously satisfying the set of these requirements has considerably impeded development of this type of engine.
Document CH-A-593 420 noted above presents only one schematic arrangement, on the basis of which it is impossible to gather any information able to result in a global technical solution satisfying the set of these requirements. In fact, it does not take into account the architectural constraints specific to an arrangement of two cylinders with parallel axes noted in 10). Constraint 8) is only partially taken into account and moreover does not allow proposal of a solution satisfying constraints 9)-12). In particular, the scavenging system presented is of transverse type with a piston head in the shape of a deflector. This scavenging system, known per se, leads to a relatively mediocre scavenging output.
Other scavenging forms of loop type, also known under the name of "Schnurle" scavenging, have been tested. They have allowed better consideration, although imperfectly, cf the constraint noted in 9), though without satisfying the other requirements, notably 8), 11), and 12).
The evolution of the scavenging technology of engines of conventional type with distribution exclusively assured by the piston in combination with exhaust ports and inlet has finally adopted, despite the large number of devices proposed, only the Schnurle loop solution noted above, which consists in admission of scavenging air into the cylinder with its impingement initially on the piston head, then on the lower part of the cylinder in immediate proximity of the piston head diametrically opposite the exhaust ports, then on the base of the cylinder head, and finally on the cylinder wall situated above the exhaust ports before undergoing an inflection in the opposite direction towards the exhaust ports.
Also known according to German Patent DE-477 041 KRUPP, cited in a book on scavenging written by VENEDIGER, printed by FRANCKHISCHE VERLAGSHANDLUNG, Abt. Technik, Stuttgart, 1947, and quoted in French Patent FR-A-769 039, is a scavenging system said to be of inverse loop type consisting in admission of scavenging air with its intended impingement on the wall of the cylinder liner situated above the exhaust ports.
This inverse loop system has not achieved any success, since it notably presents the inconvenience of causing an increase in the rising flows of scavenging air in the direction of the intended liner wall and of the flow of burnt gases emerging towards the exhaust ports, with a major contact area and insufficient impingement of scavenging air at its entry into the cylinder, while thus realizing two characteristics that should be carefully avoided if it is desired to guarantee sufficient scavenging stability (see page 73, first paragraph, of the book by VENEDIGER).
There has consequently been prejudice in the art for use of inverse loop scavenging.
In all proposed solutions proposed the frame of inverse loop scavenging, in the same way as in virtually all other known solutions, the lower edge of the inlet ports coincides with the position of the piston head at the bottom dead center (see particularly FIG. 1 of FR-769 037 or FR-E-45284).